Zinc Selenide (ZnSe)
Zinc Selenide has the lowest absorption of the common CO2 transmitting materials and is, therefore, the material of choice for high power applications. It is also the only material that transmits visible light.
Gallium Arsenide (GaAs)
Gallium Arsenide is the material of choice in dirty or high-spatter environments. It has relativity high hardness which helps it repel debris particles. It also has high thermal conductivity which helps transmit heat away from imbedded particles.
Germanium can only be used in low power applications because it is subject to thermal runaway. As temperature increases, its absorption increases, which leads to thermal failure in high power applications.